Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method for selecting a cell in a communication environment in which a plurality of wireless networks is supported.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. The 3GPP LTE adopts MIMO (multiple input multiple output) having maximum four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
FIG. 1 is a schematic diagram illustrating a structure of evolved mobile communication network.
As shown in FIG. 1, an evolved UMTS terrestrial radio access network (E-UTRAN) is connected to an evolved packet core (EPC).
The E-UTRAN includes base stations (or eNodeBs) 20 that provides a control plane and a user plane to a user equipment (UE). The base stations (or eNodeBs) 20 may be interconnected through an X2 interface.
The radio interface protocol layers between the UE and the base station (or eNodeB) 20 may be divided by L1 (a first layer), L2 (a second layer) and L3 (a third layer) based on lower three layers of open system interconnection (OSI) standard model that is widely known in communication systems. Among these layers, a physical layer included in the first layer provides an information transfer service using a physical channel, and a radio resource control (RRC) layer located at the third layer performs a role of controlling radio resources between the UE and the base station. For this, the RRC layer exchanges a RRC message between the UE and the base station.
Meanwhile, the EPC may include various elements. FIG. 1 shows a mobility management entity (MME) 51, a serving gateway (S-GW) 52, a packet data network gateway (PDN GW) 53 and a home subscriber server (HSS) 54 among the various elements.
The base station (or eNodeB) 20 is connected to the mobility management entity (MME) 51 of the EPC through an S1 interface, and is connected to the serving gateway (S-GW) 52 through an S1-U.
The S-GW 52 is an element that operates at a boundary point between a radio access network (RAN) and a core network and has a function of maintaining a data path between an eNodeB 20 and the PDN GW 53. Furthermore, if a user equipment (UE) moves in a region in which service is provided by the eNodeB 20, the S-GW 52 plays a role of a local mobility anchor point. That is, for mobility within an E-UTRAN (universal mobile telecommunications system (Evolved-UMTS) terrestrial radio access network defined after 3GPP release-8), packets can be routed through the S-GW 52. Furthermore, the S-GW 52 may play a role of an anchor point for mobility with another 3GPP network (i.e., a RAN defined prior to 3GPP release-8, for example, a UTRAN or global system for mobile communication (GSM) (GERAN)/enhanced data rates for global evolution (EDGE) radio access network).
The PDN GW (or P-GW) 53 corresponds to the termination point of a data interface toward a packet data network. The PDN GW 53 can support policy enforcement features, packet filtering, charging support, etc. Furthermore, the PDN GW (or P-GW) 53 can play a role of an anchor point for mobility management with a 3GPP network and a non-3GPP network (e.g., an unreliable network, such as an interworking wireless local area network (I-WLAN), a Code Division Multiple Access (CDMA) network, or a reliable network, such as WiMax).
In the network configuration of FIG. 1, the S-GW 52 and the PDN GW 53 have been illustrated as being separate gateways, but the two gateways may be implemented in accordance with a single gateway configuration option.
The MME 51 is an element for performing the access of a terminal to a network connection and signaling and control functions for supporting the allocation, tracking, paging, roaming, handover, etc. of network resources. The MME 51 controls control plane functions related to subscribers and session management. The MME 51 manages numerous eNodeBs 22 and performs conventional signaling for selecting a gateway for handover to another 2G/3G networks. Furthermore, the MME 51 performs functions, such as security procedures, terminal-to-network session handling, and idle terminal location management.
Meanwhile, recently, the high speed data traffic has been rapidly increased. In order to meet such traffic increase, technologies have been introduced for offloading the traffic of UE to WLAN (Wi-Fi) or a small cell.
FIG. 2 is a schematic diagram illustrating a network structure to which a small cell or a WLAN AP is added.
Referring to FIG. 2, within the coverage of a base station 31 for the small cell, a plurality of WLAN AP may be arranged. That is, several radio access technologies (RATs) are existed around a UE. Accordingly, the UE may distribute data traffic into the several RATs. The base station 31 for small cell may be arranged within the coverage of a macro base station such as the existing eNodeB.
As known from by reference to FIG. 2, the P-GW 53 and the HSS 54 are connected to an access authentication authorization (AAA) server 56. The P-GW 53 and the AAA server 56 may be connected to an evolved packet data gateway (ePDG) 57. The ePDG 57 plays a role of a security node for not being trusted non-3GPP network (e.g., WLAN or Wi-Fi, etc.). The ePDG 57 may be connected to a WLAN access gateway (WAG) 58. The WAG 58 may be in charge of a role of P-GW in Wi-Fi system.
As such, as the existing mobile communication network is associated with a hetero network, a discussion is required for selecting an optimal wireless network for performing a specific operation.